1. Field of the Invention
The invention is generally directed to assemblies for transmission lines that include a heat sensitive dielectric, and is more particularly directed to methods and apparatuses for coupling transmission lines to circuit boards.
2. Description of the Related Art
Various schemes for coupling transmission lines to circuit board assemblies are known in the prior art. Many of such prior art schemes fall into one of three basic categories:
1. In well known connector type schemes, various connectors for use at high frequency (defined herein substantially within a range from approximately one Mega-Hertz to approximately five Giga-Hertz, so as to also include ultra-high frequency), for example BNC, SMA, SMB, TNC, MCX, MMCX, N, F, UHF and Mini UHF type connectors, well known in the art, are employed. For the connector scheme, the transmission line is coupled to half of the connector system, another half of the connector system is soldered to the printed circuit board, and then both halves of the connector are mated to provide electrical connection.
2. In a manual direct soldering scheme, the transmission line is soldered directly (usually by hand) to plated areas (pads or vias) in the printed circuit board.
3. In an automated direct soldering scheme, conductors of the transmission line and their corresponding mating pads on the printed circuit board are each tinned or provided with a supply of solder, usually in the form of solder paste. The conductors of the transmission line are held in place on their mating pads with fixturing, while a heat source (anvil, hot air, or LASER) is used to reflow the joints.
Advantages of the first scheme are primarily limited to those situations where multiple cycles of connecting and disconnecting the transmission line from the circuit board are needed. However, in many cases only the connection is needed. Disconnection isn't needed or desired. Disadvantages of the connector scheme include the complexity, unreliability, ohmic signal loss, cost and size of the mating connector. For extremely small printed circuit board assemblies, large connectors are particularly undesirable. For example, with extremely small circuit boards (such as those on the order of approximately a half an inch square) used in oscilloscope probe assemblies, the size of a connector may rival or exceed that of the circuit board.
The second scheme of manual direct soldering provides advantageous low ohmic signal loss. The disadvantages of this scheme include the time consuming, expensive manual labor, and further include undesirable variability of hand soldered connections. Also, it is usually necessary, when working with a multistranded conductor of a transmission line, for example an outer braid conductor of a coaxial cable, to prepare an extremity of the multistranded conductor with twisting and/or tinning to keep the individual strands from unraveling. These processes are usually done by hand, and suffer from the same variability and time consuming aspects as any other hand operation. Additionally, care must be used in hand soldering when the transmission line includes heat sensitive dielectric.
The third scheme is an attempt to automate the manual direct soldering of the second scheme. Process variables and difficulties in holding conductors of the transmission lines in place make this scheme only slightly better than hand soldering. When the transmission line includes heat sensitive dielectric, care must be used with the heat source (anvil, hot air, or LASER) used to reflow the joints.
The heat sensitive dielectric, as defined herein, is one that may be substantially deformed or otherwise damaged if exposed to excessive heat during the soldering. Such deformation or other damage caused by such excessive heat may interfere with insulation, for example, the dielectric insulating a first conductor of the transmission line from an additional conductor of the transmission line, resulting in a short circuit. Alternatively, such deformation or damage of the heat sensitive dielectric may introduce a lesser electrical discontinuity for high frequency signals, that still substantially interferes with signal integrity.
Examples of such heat sensitive dielectrics are included in the following table. In the table, following each name is a respective approximate maximum service temperature. The approximate maximum service temperature is a recommended temperature that the particular dielectric material can withstand. At a substantially higher temperature, deformation, melting, or even scorching of the dielectric may take place. Of course, there is variation in these temperatures based on factors such as particular formulations. However the temperatures summarized in the table are generally considered to be fairly representative in the literature, such as plastics encyclopedias.
It should be particularly noted that all temperatures for the examples of heat sensitive dielectrics in the table are well below that a melting point of a typical solder (for example a eutectic solder having a composition of 63% Tin (Sn) and 37% Lead (Pb) has a melting point of three-hundred and sixty-one degrees Fahrenheit (361 F).) Accordingly, it should be understood that excessive heat of soldering poses a real and substantial threat to integrity of heat sensitive dielectrics.